Gas sensors which give a warning before gases leaking into the atmosphere reach a dangerous level and cause an explosion or become harmful to the human body have been widely known for a considerable time. In particular, carbon monoxide is dangerous to the human body or other living organisms even when it is present in air in trace amounts that are far smaller than the amounts required to cause an explosion, and it is therefore essential to have something that will generate an alarm when carbon monoxide is present at several hundred ppm.
Hitherto, technology employing ceramic semiconductor substances has been developed for gas sensors for this purpose. For example, a detailed account of this technology is given in:
Citation 1: Miyayama and Yanagida: "A Zinc Oxide Gas Sensor" Seramikkusu (Ceramics Japan), Vol. 18, No. 11 (November 1983), pp. 941.about.945, published by the Ceramic Society of Japan PA1 Citation 2: Miyayama and Yanagida: "New Developments in Gas Sensor Materials" Denki Kagaku (Electrochemistry), Vol. 50, No. 1 (January 1982), pp. 92.about.98 PA1 Citation 3: Yanagida et al.: "Current-Voltage Characteristics of Semiconductor Junctions as Functions of Relative Humidity" Japanese Journal of Applied Physics, Vol. 22, No. 12, (December 1983), p. 1933 PA1 Citation 4: Japanese Patent Application Disclosure No. 62-90529. PA1 Citation 5: Japanese Patent Application Disclosure No. 62-90528. PA1 (1) a stable p-n contact region is formed and stable characteristics are exhibited; PA1 (2) because the thick film or the substrate is porous, the gas to be detected is introduced efficiently to the p-n contact region; PA1 (3) because the area of contact between the p-n contact region and the gas is increased, good characteristics are exhibited.
This technology utilizes the property that if a reducing gas comes into contact with the surface of a ceramic semiconductor, the height and width of the potential barrier will decrease as a result of the adsorbed oxygen that is present at the surface of the semiconductor reacting with the gas, which will lead to movement of electrons becoming easier and resistivity decreasing.
One of the inventors of the invention disclosed in this application also noticed that the rectifying junctions of a metal and a semiconductor, or of two different ceramic semiconductors, react with hydrogen gas or steam, and has suggested that a promising technique for the future would be to utilize changes in rectifying characteristics for the detection of hydrogen or water vapour in the air. An account of this appears in:
or;
Although one of the inventors of the invention disclosed in this application suggested in the above-noted well-known publications that semiconductor junctions with rectifying characteristics would be effective for the detection of hydrogen gas and water vapour in the air, at that stage the working of such a device had not been fully elucidated, and so it was not clear what kinds of gases could be detected, nor what techniques or device configurations could be utilized for practical applications.
Some of the applicants of the present application have previously filed a patent application for a gas detection method and a gas sensor which can selectively detect target gases at low temperature, and this application has been laid open to public inspection as:
This publication teaches (i) bringing into mechanical contact, by way of a contact surface, two types of solid substance which have rectifying properties when brought into mutual contact, (ii) forming gaps at this contact surface, and then (iii) introducing a sample gas into these gaps. Given this arrangement, the type of gas to which the device is sensitive varies with change in bias voltage. This is thought to be because the energy level of the potential barrier at the surface of these semiconductor substances changes to a different value.
The aforementioned group of applicants also filed a patent application for the detection of carbon dioxide using a construction equivalent to that set forth in Citation 4, and this application has been laid open to public inspection as:
The object of the present invention is to provide gas sensors which represent a further improvement of the gas sensors disclosed in Citations 4 and 5 above, which have practical characteristics, and which are suited to mass production. More specifically, the object of the present invention is to provide gas sensors which exhibit stable characteristics and with which good sensor performance is obtained by having (i) improved circulation of the sample gas around the contact region and (ii) increased area of contact between the semiconductor contact region and the gas.